Move socket watch APIs into System::Layer

[kpschoedel]

Problem

Transitionally, System::Layer operations are provided by a
configuration-specific WatchableEventManager, but they are not yet
sufficiently aligned to convert to abstract and implementation classes.

Part of #7715 Virtualize System and Inet interfaces

Change overview

• Move socket event watch APIs into System::Layer.
• Remove WatchableSocket and its include-file hack that allowed
  implementation-specific state to be contained directly in EndPoints.
• Revise the select() implementation to use an internal pool for watch
  state in place of the EndPoint-embedded WatchableSocket.
• Revise the libevent sample implementation to use libevent timers
  directly instead of the System::Timer pool. The libevent
  implementation uses STL containers with heap-allocated state for the
  sake of being different from the select()-based implementation.
• Added/moved documentation comments into headers.

Testing

CI; no change to functionality should is expected.

[github-actions]

Size increase report for "gn_qpg-example-build" from 3acbf59

File	Section	File	VM
chip-qpg6100-lighting-example.out	.text	32	32

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-qpg6100-lighting-example.out and ./pull_artifact/chip-qpg6100-lighting-example.out:

sections,vmsize,filesize
.debug_loc,0,227
.strtab,0,126
.debug_abbrev,0,74
.symtab,0,64
.debug_frame,0,32
.text,32,32
.debug_aranges,0,8
.debug_ranges,0,8
.shstrtab,0,2
.debug_str,0,-4
[Unmapped],0,-32
.debug_info,0,-374
.debug_line,0,-1127

Comparing ./master_artifact/chip-qpg6100-lighting-example.out.map and ./pull_artifact/chip-qpg6100-lighting-example.out.map:

BLOAT EXECUTION FAILED WITH CODE 1:
bloaty: unknown file type for file './pull_artifact/chip-qpg6100-lighting-example.out.map'

[github-actions]

Size increase report for "nrfconnect-example-build" from 3acbf59

File	Section	File	VM
chip-shell.elf	[LOAD #3 [RW]]	0	29
chip-shell.elf	device_handles	4	4
chip-shell.elf	text	-20	-20
chip-shell.elf	bss	0	-29
chip-lock.elf	device_handles	4	4
chip-lock.elf	text	-20	-20
chip-lock.elf	bss	0	-32

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_loc,0,1390
.strtab,0,152
[LOAD #3 [RW]],29,0
.debug_aranges,0,8
.debug_ranges,0,8
device_handles,4,4
.symtab,0,-16
text,-20,-20
bss,-29,0
.debug_frame,0,-48
.debug_abbrev,0,-1115
.debug_str,0,-1746
.debug_line,0,-2466
.debug_info,0,-23711

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_loc,0,1566
.strtab,0,152
.debug_aranges,0,8
.debug_ranges,0,8
device_handles,4,4
.symtab,0,-16
text,-20,-20
bss,-32,0
.debug_frame,0,-48
.debug_abbrev,0,-1216
.debug_str,0,-1746
.debug_line,0,-7242
.debug_info,0,-40570

[github-actions]

Size increase report for "esp32-example-build" from 3acbf59

File	Section	File	VM
chip-lock-app.elf	.flash.text	52	52
chip-all-clusters-app.elf	.flash.text	32	32
chip-bridge-app.elf	.flash.text	4	4
chip-temperature-measurement-app.elf	.flash.text	52	52
chip-shell.elf	.flash.text	88	88

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize
.debug_abbrev,0,175
.strtab,0,126
.flash.text,52,52
.symtab,0,32
.debug_frame,0,24
.debug_aranges,0,8
.debug_ranges,0,8
.shstrtab,0,-2
[Unmapped],0,-52
.debug_info,0,-403
.debug_loc,0,-404
.debug_line,0,-876

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize
.debug_str,0,-7
.debug_info,0,-8
.debug_line,0,-17

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_loc,0,620
.debug_abbrev,0,177
.strtab,0,126
.flash.text,32,32
.symtab,0,32
.debug_frame,0,16
.debug_aranges,0,8
.debug_ranges,0,8
.riscv.attributes,0,1
.debug_str,0,-2
.shstrtab,0,-2
[Unmapped],0,-32
.debug_info,0,-213
.debug_line,0,-843

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize
.debug_abbrev,0,220
.strtab,0,126
.symtab,0,32
.debug_frame,0,24
.debug_aranges,0,8
.debug_ranges,0,8
.flash.text,4,4
.debug_str,0,1
.shstrtab,0,-2
[Unmapped],0,-4
.debug_loc,0,-297
.debug_info,0,-443
.debug_line,0,-853

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize
.debug_str,0,-7
.debug_info,0,-8
.debug_line,0,-17

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize
.debug_abbrev,0,212
.strtab,0,126
.debug_loc,0,67
.flash.text,52,52
.symtab,0,32
.debug_frame,0,24
.debug_aranges,0,8
.debug_ranges,0,8
.shstrtab,0,2
.debug_str,0,-3
[Unmapped],0,-52
.debug_info,0,-455
.debug_line,0,-853

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_abbrev,0,171
.strtab,0,126
.flash.text,88,88
.symtab,0,32
.debug_frame,0,24
.debug_aranges,0,8
.debug_ranges,0,8
.shstrtab,0,2
.debug_str,0,1
[Unmapped],0,-88
.debug_loc,0,-228
.debug_info,0,-343
.debug_line,0,-445

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize
.debug_str,0,15
.debug_info,0,-16
.debug_line,0,-51

[woody-apple]

@saurabhst ?